SEIF 3

If someone's morning meal stretches until Mincha time, he can split the meal in two in order to fulfill the Mitzvah of Shaalos Seudos. The Rosh explains that this it is advisable to do so, since  if one were to eat a large meal, he might be too full to eat Shaalos Seudos at a later time. Rather, he should split up the meal and eat while he still has his appetite. (Sometimes a person can eat a large meal at one time, but waiting until later might ruin his appetite and he would be too full to eat another meal.)

The Magen Avraham in the name of the Mahril says that one needs to get up and take a walk between the two meals. Since eating the second meal immediately following  the first would not be considered as having one continuous meal. The Elya Rabba in the name of Mahrshal says that one should take a break between the meals by speaking in Torah or some other type of Bitul.

The Bais Yosef quotes the Rosh that a Rav should not announce that this in his Halacha Drasha in order to make a break between the two meals, since the average layman is not expert enough to know the difference between splitting the meal before or after the time for Mincha.

The Ri asks why doesn't splitting a meal in two cause extra, unnecessary Brachos, since it would entail Bentching and making another Hamoitzie.

The Rosh answers that since this is a requirement in order to be Yoitze Shaalos Seudah, the Brachos cannot be referred to as unnecessary. The Rama says that this Heter should only be used when a person feels he may not be able to eat Shaalos Seudah at the proper time, but if it is possible to eat later, one should preferably wait until after Mincha to eat Shaalos Seudos. The Magen Avraham explains this is because L'chatchila we should try to accommodate the Ri's Shita (i.e. that it causes unnecessary Brachas).